add vram control register read handling

git-svn-id: https://svn.code.sf.net/p/desmume/code/trunk@4787 5577840e-afb3-4e59-9ef3-2b7f898b1952
revert r4787. this was already supposedly handled by MMU_VRAMmapControl(). there may still be bugs in mmu.cpp which blocked that from working

git-svn-id: https://svn.code.sf.net/p/desmume/code/trunk@4788 5577840e-afb3-4e59-9ef3-2b7f898b1952
remove logic from mmu.cpp which might have been interfering MMU_VRAMmapControl()

git-svn-id: https://svn.code.sf.net/p/desmume/code/trunk@4789 5577840e-afb3-4e59-9ef3-2b7f898b1952